text:
Which parts of class E airspace can an ultralight (part 103) fly in without prior ATC authorization?
I'd like to clarify a very specific statement from 14 CFR Part 103:

103.17 OPERATIONS IN CERTAIN AIRSPACE
  No Person may operate an ultralight vehicle within Class A, Class B, Class C, or Class D airspace or within the lateral boundaries of the surface area of Class E airspace designated for an airport unless that person has prior authorization from the ATC facility having jurisdiction over that airspace.

The specific statement I need help with is:

or within the lateral boundaries of the surface area of Class E airspace designated for an airport

As we know, Class E airspace can start at the surface (shown as a dashed magenta line on sectional charts), start at 700' AGL (shown as a magenta border), start at 1,200' AGL (shown as a blue border), or start at 14,500 MSL.
In the Aeronautical Information Manual (AIM), the following relevant information is given about Class E airspace under section 3-2-6:

e. Functions of Class E Airspace. Class E airspace may be designated for the following purposes:

Surface area designated for an airport where a control tower is not in operation. Class E surface areas extend upward from the surface to a designated altitude, or to the adjacent or overlying controlled airspace. The airspace will be configured to contain all instrument procedures.

(a) To qualify for a Class E surface area, the airport must have weather observation and reporting capability, and communications capability must exist with aircraft down to the runway surface.
(b) A Class E surface area may also be designated to accommodate part-time operations at a Class C or Class D airspace location (for example, those periods when the control tower is not in operation).
(c) Pilots should refer to the airport page in the applicable Chart Supplement U.S. for surface area status information.

Extension to a surface area. Class E airspace may be designated as extensions to Class B, Class C, Class D, and Class E surface areas. Class E airspace extensions begin at the surface and extend up to the overlying controlled airspace. The extensions provide controlled airspace to contain standard instrument approach procedures without imposing a communications requirement on pilots operating under VFR. Surface area arrival extensions become part of the surface area and are in effect during the same times as the surface area.

Part 103 states "or within the lateral boundaries of the surface area of Class E airspace designated for an airport".
That statement has two parts I need help with:

within the lateral boundaries of

and

the surface area of Class E airspace designated for an airport

When looking at a sectional and reading the relevant section of the AIM, what exactly applies to part 103?
I've seen a few attempts to answer the following questions in other forums and it always ends up in a debate including conflicting info from pilots and even FSDO personnel. Is there a clear, definitive "aviation lawyer" legal type answer to the following questions (I'm not concerned with any "should" type discussions here)?
Question 1:
What exactly is the surface area of Class E airspace designated for an airport? Is it any dashed magenta area on a sectional or is it something more specific or different?
Question 2:
And does the within the lateral boundaries of part mean there is no ceiling (meaning an ultralight can't overfly these areas at any altitude) or something else?
For example (for Q3&4), BLYTHE (BLH) (Phoenix sectional) airport:

Question 3:
Can an ultralight fly within the dashed magenta circle? Is this an example of a "surface area of Class E airspace designated for an airport"?
Question 4:
If this is true, what is the ceiling that affects ultralight aircraft? Is it the 700' AGL Class E airspace above the airport? Or does it go all the way up to FL180? In other words, can an ultralight overfly this airport as long as it flies more than 700' AGL?
And what about (for Q5&6) Yuma (NYL) (Phoenix sectional) airport:

Question 5:
Can an ultralight fly within the two Class E extensions to the Class D airport? Or are these two extensions still considered "surface areas of Class E airspace designated for an airport" (and this has the same answer as question 1 above)?
Question 6:
If an ultralight can't fly within these two Class E extensions, can an ultralight flow over these extensions? Is the ceiling 2,700' MSL (same as the Class D airspace of the airport) or is the ceiling 700' AGL (due to the overlying Class E airspace in the area)? Or does it go to FL180 (which would make no sense since an ultralight can overfly the Class D airspace)?

Related

@Pondlife I rolled back your title edit because it completely changed the focus of my question.

This is really a multi-part question that arguably would have been better asked as several separate questions.  Note also that there are many FARs other than the ultralight-specific FAR 103.17 that are affected by the interpretation of the meaning of "within the lateral boundaries of the Surface Area..."  -- see linked questions below for more on this.  Also note that this issue also arises in reference to Class B and Class C airspace, not just in reference to surface-level Class E airspace.

The specific question about how high above the airport the off-limits area extends, has now been addressed in a more focused question https://aviation.stackexchange.com/q/74979/34686 .  See also https://aviation.stackexchange.com/q/74384/34686 (really a duplicate, which I tried to delete but wasn't allowed to.)

The issue of whether the extensions (see Yuma illustration) fall within the meaning of the phrase "within the lateral boundaries of the Surface Area" is its own, surprisingly "sticky" issue; I've limited my (primary) answer here to focus on that specific issue.

NOTE: This answer was originally given based on a conservative interpretation of available interpretations of the regulations. New guidance may shed new light on the issue. The interpretation presented here may be superseded; see the discussion and other answers.

Short Answer

within the lateral boundaries of the surface area of Class E airspace designated for an airport

This refers to the horizontal dimensions of Class E airspace that contacts the surface of the earth without any reference to vertical dimensions or ceiling.

Long Answer
You ask for definitive and official clarification on the following two portions of §103.17:

within the lateral boundaries of

and

the surface area of Class E airspace designated for an airport

Here is §103.17 in full for context:

Operations in certain airspace.
No person may operate an
ultralight vehicle within Class A, Class B, Class C, or Class D
airspace or within the lateral boundaries of the surface area of Class
E airspace designated for an airport unless that person has prior
authorization from the ATC facility having jurisdiction over that
airspace.

I will address these two clauses separately based on the official legal interpretations, then address your six case questions based on the definitions established. Throughout this answer I will also assume that no "prior authorization" has been obtained from ATC that would give exception to the above regulation.

"within the lateral boundaries of..."
The terminology, "the lateral boundaries" refers to the horizontal limits of an area in question; the terminology does not refer to any vertical limits.
Because "lateral" defines only horizontal area—not vertical—the terminology "within the lateral boundaries" means within the horizontal area defined, without respect to any vertical bounds, unless otherwise specified.
Where a regulation needs to define vertical limits, additional terminology is used, such as "below [altitude]" or "above the ceiling". Where the totality of an airspace is intended, a comprehensive phraseology is used, such as "within the lateral and vertical boundaries".
Compare the ultralight rule of §103.17 with wording of the following regulations respectively governing unmanned free balloons and transponder use—both of which define the horizontal limits of the airspace in question by the same terminology "within the lateral boundaries of":

§101.33 (a) Unless otherwise authorized by ATC, below 2,000 feet above the surface within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for an airport;

§91.215 (4) All aircraft in all airspace above the ceiling and within the lateral boundaries of a Class B or Class C airspace area designated for an airport upward to 10,000 feet MSL; and

"...the surface area of Class E airspace..."
Note: Credit goes to Pondlife for identifying the legal interpretation—Hucker (2006)—that addresses this issue.
The FAA has defined this terminology very clearly and without ambiguity.
The following definition of "surface area" is provided in the Pilot Controller Glossary:

SURFACE AREA− The airspace contained by the lateral boundary of the
Class B, C, D, or E airspace designated for an airport that begins at
the surface and extends upward.

Tthe FAA has issued a letter of legal interpretation—Hucker (2006)—that definitively addresses this:

The term “surface areas” refers only to those components of airspace that come in contact with the surface of the earth.

Moreover, Hucker (2006) addresses the regulations of §91.303 pertaining to aerobatic flight (which contains some of the same language as §103.17):

§91.303
(c) Within the lateral boundaries of the surface areas of Class B,
Class C, Class D, or Class E airspace designated for an airport;

In addressing this regulation, Hucker (2006) makes clear that the above defines lateral areas without any vertical limitation.

"...designated for an airport"
I would argue that this phraseology is superfluous and that the FAA also regards it as such. I know of no Class B, C, D, or Class E airspace that extends to the surface that is not designated for an airport. I would be happy to be shown wrong, however.
As I said, I believe the FAA also views this language as superfluous:
In the FAA's ATC order JO 7110.65W the rules for SVFR are addressed in reference to the applicable airspaces, but without differentiation as to whether or not such airspaces are designated for an airport:

7-5-1
2. Only within the lateral boundaries of Class B, Class C, Class D, or Class E surface areas, below 10,000 feet MSL.

Additionally, in a memorandum of legal interpretation, Duncan (2015), the airspeed rules of §91.117 for Class B airspace are addressed, also without differentiation as to whether or not the Class B airspace is designated for an airport.

Given the above, I would answer your six questions as follows:

Question 1:
What exactly is the surface area of Class E airspace designated for an airport? Is it any dashed magenta area on a sectional or is it something more specific or different?

Yes, the surface area of Class E airspace is the component of Class E airspace that contacts the surface of the earth and extends upward.

Question 2:
And does the within the lateral boundaries of part mean there is no ceiling (meaning an ultralight can't overfly these areas at any altitude) or something else?

Yes, in the context of §103.17 where no vertical dimensions are specified, within the lateral boundaries means inside the horizontal area with no vertical limitation or ceiling.

Question 3:
Can an ultralight fly within the dashed magenta circle? Is this an example of a "surface area of Class E airspace designated for an airport"?

No, without prior authorization, no person may operate an ultralight vehicle within the lateral boundaries of the surface area of Class E airspace designated for an airport. Yes, this is an example of Class E airspace designated for an airport.

Question 4:
If this is true, what is the ceiling that affects ultralight aircraft? Is it the 700' AGL Class E airspace above the airport? Or does it go all the way up to FL180? In other words, can an ultralight overfly this airport as long as it flies more than 700' AGL?

Since §103.17 does not define any vertical limitation, no person may operate an ultralight vehicle anywhere within the lateral boundaries of the surface area of Class E airspace designated for an airport. This prohibition, includes any airspace vertically overlying the Class E surface area, including the Class A airspace at and above FL180.

Question 5:
Can an ultralight fly within the two Class E extensions to the Class D airport? Or are these two extensions still considered "surface areas of Class E airspace designated for an airport" (and this has the same answer as question 1 above)?

No, these areas are surface areas of Class E airspace designated for an airport.

Question 6:
If an ultralight can't fly within these two Class E extensions, can an ultralight flow over these extensions? Is the ceiling 2,700' MSL (same as the Class D airspace of the airport) or is the ceiling 700' AGL (due to the overlying Class E airspace in the area)? Or does it go to FL180 (which would make no sense since an ultralight can overfly the Class D airspace)?

No, an ultralight must not be flown over these extensions.

Thank you. This answer agrees with my own understanding. But I was hoping I was wrong. Every other use of "lateral boundary" that I saw in other regs made mention of altitudes just as in your examples. And since 103.17 doesn't, it appears to indicate a needless restriction for ultralights. Why can I fly over a class D airspace but not over an associated class E surface extension to that class D airspace, for example?

@rmaddy I agree that the regulation seems unnecessarily restrictive when it comes to class E airspace. However, neither do I think that it would make sense to prohibit ultralight flight from the surface to 700 AGL, but allow flight in that same lateral area above 700 AGL, which alternate interpretations would suggest.

Dug into this more, I'm not sure any more whether I agree with the "lateral boundary" part of this answer. Here's why. Sections 91.215(b)(5)(ii), 91.255(d)(3), 101.33(a), and 135.205(b) all use the phrase "the lateral boundaries of" as well as clear altitude delineations. Those are clear. Then there is section 91.303 (aerobatics) which uses the same term. But the only altitude reference is staying above 1500' AGL. We know this means you are allowed to do aerobatics under the outer rings of class B above 1500' AGL. But can you do aerobatics over an airport, above its airspace? Continued...

And of course section 103.17 with no specific altitude references. So now I can't decide whether a lack of complete altitude references along with "the lateral boundaries of" means surface to outer space or it only means within the altitude range containing the referenced airspace.

@rmaddy §91.303(c) states "within the lateral boundaries of the surface areas of Class B". If this meant only that portion of the surface area airspace that underlies any above airspace, it would be mostly redundant since §91.303(e) already prohibits aerobatics below 1500 AGL.

@rmaddy And note the lack of vertical distinction in this sentence from Hucker (2006): "When the floor of Class D and certain Class E airspace (designated for an airport) begins at the surface and extends upward, aerobatics are prohibited in accordance with section 91.303(c)." No vertical limitation is given. This is in contrast to the Class E shelves that are not surface areas.

I'm not sure how what you stated answers my question. Can, for example. aerobatics be done legally above a class D airport, above the ceiling of the class D airspace? I spent hours searching the Internet trying to find a definitive answer to that. And the "Hucker" discussion does not clarify that for me.

And similarly, 91.303 clearly means that aerobatics can be done above the outer rings of class B airspace. But it is unclear if aerobatics can be done above the center core ("within the later boundaries of the surface area"). I think a definitive answer to that would answer my 103.17 question.

@rmaddy I think my answer here addresses that clearly: within the lateral boundaries means—in plain English, by inference from the regulations and the FAA's interpretation—within the two dimensional space defined by the boundaries without reference to any vertical dimension.

@rmaddy Unless an FAA interpretation or regulation clearly overturns the plain meaning of the English, the regulations stand as written.

Let us continue this discussion in chat.

Re the reference in this answer to "The FAA has issued a letter of legal interpretation—Hucker (2006)—that definitively addresses this:"--  a) can someone edit this answer (and/or post a comment) to give a working link to this document-- and b) just because the ruling makes it clear that only the airspace in the region where the floor goes all the way down to the ground is included in the concept of "within the lateral boundaries of the surface area of Class E airspace designated for an airport", doesn't mean that ALL the Class E whose floor touches the ground is included by that language.

Thank you for the fresh link to Hucker 2006.  They sure messed up with their original interpretation didn't they.  However note that just because the definition of "surface area" does not have anything to do with the parts of the Class B airspace that do NOT touch the ground, doesn't mean that definition includes ALL controlled airspace in contact with the ground, particularly in the case of Class-E-to-surface where it is clear from the regs, JO 7700, and AIM that there really are two different kinds of Class-E-to-surface, and only one gets the label "Surface Area", as per my answer.  .

Also you said "In the FAA's ATC order JO 7110.65W the rules for SVFR are addressed in reference to the applicable airspaces, but without differentiation as to whether or not such airspaces are designated for an airport: 7-5-1 2. Only within the lateral boundaries of Class B, ... or Class E surface areas, below 10,000 feet MSL."  This does seem a grey area to me.  Based on the language of the AIM and the SECTIONS HEADINGS of JO 7400, I think we could make the case that the mere phrase "Surface Area" alone is sufficient to indicate that we are NOT talking about Class-E-to-surface "extensions".

Also regarding "Additionally, in a memorandum of legal interpretation, Duncan (2015), the airspeed rules of §91.117 for Class B airspace are addressed, also without differentiation as to whether or not the Class B airspace is designated for an airport."-- A) can you provide a fresh link to this too?  and B) since all Class B is "designated for an airport", it seems to me the lack of significance of the presence or absence of this phrase for Class B doesn't tell us anything about the same for Class E.

I looked some more at 2006 Hucker ruling--disappointing. Narrowly missed a chance to clarify whether or the Class-E-to-surface extensions fall w/i meaning of "lateral boundries of the srfce areas of... Class E airspce dsgntd for an airport".I don't think they even realized it. They talk about one kind of class E that goes to the surface and is "designated for an airport" and they then raise the issue of another kind of Class E but it turns out all they mean is the transition areas with the higher floors.
Read more: http://forum.hanggliding.org/viewtopic.php?t=19101&start=180#ixzz5TantOLZq

@quietflyer I don't have time at present to delve into the details of this issue. The new memorandum you presented does seem to shed new light on the issue. At present I have left a note at the top of my answer pointing to these new developments.

Coming back to this issue once more-- my view now is that in the specific context of surface-level class E airspace, and more specifically, the "extensions", the significance of phrases like "surface area", "designated for an airport", and "surface area of Class E airspace designated for an airport" or "surface area of controlled airspace designated for an airport" is ambiguous, and is not resolved by any of the cases cited in your answer.  However I no longer put so much stock in other recent memos pertaining to drones/ unmanned aircraft etc as it's become clear that those views are not

However I no longer put so much stock in other recent memos pertaining to drones/ unmanned aircraft etc-- or in the letter from the Western ATO office re not authorizing SVR in surface-level Class E "extensions"-- as it has  become clear that those views are not shared universally throughout the FAA. The situation remains unresolved in my opinion.

The definitive source for what airspace is where for airports is contained in an FAA Order.  That order is updated periodically, and has numerous amendments between issuances.  The current order is JO 7400.11B.  The order is exhaustive and states on page A-1 that any airspace not defined within is by default Class G.
I submit that your answer is more precisely defined by the above referenced order, and not specifically by a sectional, as the sectional is a derived document from the order.
Using the OP example of Yuma, Appendix E of the order says:

AWP AZ E2 Yuma, AZ Yuma MCAS-Yuma International Airport, AZ (lat.
  32°39'24''N., long. 114°36'22''W.) Somerton, Somerton Airport, AZ
  (lat. 32°36'03''N., long. 114°39'57''W.) That airspace, within a
  5.2-mile radius of Yuma MCAS/Yuma International Airport, excluding that airspace from the surface up to and including 300 feet above the
  surface from lat. 32°36'52'' N., long. 114°41'44'' W.; thence east to
  lat. 32°36'52'' N., long. 114°39'30'' W.; thence south to lat.
  32°34'55'' N., long. 114°39'30'' W.; thence clockwise along the
  5.2-mile radius to the point of beginning. The Class E airspace area is effective during the specific dates and times established in
  advance by a Notice to Airmen. The effective date and time will
  thereafter be continuously published in the Airport/Facility
  Directory.

Since the effective date and time are published in the AFD, you would refer to that publication, and also any subsequent NOTAMs. BUT since that description has an exclusion up to 300 feet, you would exclude that area, as Class E does not begin at the surface at those exclusion areas.
A couple of things to keep in mind:

You can overfly with ATC approval.
Class E extends to overlying controlled airspace.  I guess that could be Class  (A outside continental area?), B, C or Class E beginning at 700 or 1200. It is most often Class E beginning at 700 feet AGL.
When Class E extends to the surface, it is ALWAYS associated with an airport, and there are a list of conditions which apply (WX reporting, com, etc.)
The Class E description is always in the order or addendums.

After statement "Using the OP example of Yuma, Appendix E of the order says:", you need to explain that description that follows lies within a section of JO 7400 that is titled as follows: "Class E Airspace Areas Designated as an Extension to a Class D or Class E".  Unlike the section that describes the OTHER kind of Class-E-to-surface airspace, that section does NOT bear the title "Class E Airspace Areas Designated as a Surface Area", nor is the title of that section followed by the descriptive sentence "The Class E airspace areas listed below are designated as a surface area for an airport""

Including those details would make your reference to JO 7400 much more meaningful-- but it also might force a substantial rewriting of your answer and even a change in your ultimate conclusions.  This statement in particular would need to be deleted-- "When Class E extends to the surface, it is ALWAYS associated with an airport"

Related--
(Q) What indication has the FAA given that phrases like "surface area of Class E airspace designated for an airport" do or don't include E4 "extensions"? ,
(A) What indication has the FAA given that phrases like “surface area of Class E airspace designated for an airport” do or don't include E4 “extensions”?,
and
(A) In the US, how does the distinction between E2 and E3/E4 airspace affect pilots?
In short, the situation is ambiguous as to whether ultralights may fly in the "extensions" without "prior authorization".
What is not ambiguous, however, is that if we answer "no", then we must recognize that the airspace we are talking about extends upwards indefinitely, due to the "within the lateral boundaries of" language used in the regulation. The same is true of E2 "surface area" airspace (example KONP), which is not an "extension" and which is unambiguously within the scope of the airspace addressed by the regulation (103.17) and therefore off-limits (without prior authorization) for ultralight flight.
The idea that the off-limits airspace somehow has a cap at 700' AGL or 1200' AGL, or in the case of an "extension", at the ceiling of the adjoined Class D airspace, is not supportable at all.
The truth is that the FAA is deeply divided as to whether phrases in the FARs like "surface area", "designated for an airport", and "surface area of controlled airspace designated for an airport" should or should not be construed to include surface-level Class E "extensions".1  Ambiguity around this issue has existed ever since the 1993 "alphabet" airspace re-designation.2  There's some consideration of a future rule-making effort to address this issue.3  It's possible that the language in AIM 3-2-6, as well as in the legend of the Chart Supplements, that reads "Surface area arrival extensions become part of the surface area and are in effect during the same times as the surface area" will be changed in the future to avoid implying that all regulations that apply to the core "surface area" should automatically be construed to apply to the surface-level Class E "extensions" as well.4
Footnotes:
1,2,3,4) Source-- comments made by high-level FAA staffers during April 2021 airspace charting meeting.

Related: https://www.ushpa.org/page/class-e-extensions-article .  I've since learned that this article is based on a conference call including a high-level FAA staffer who assured USHPA board members that the principles of the "Gardner memo" extended to Part 103 operations and E3/E4 "extensions" were indeed not considered to be "surface areas of Class E airspace designated for an airport" in the context of FAR oart 103 operations.  This answer needs to be reworked to include that information.

However see my cautions in this answer re statements made at April 2021 airspace charting meeting, which would have been subsequent in time to this conference call.  I would be very cautious about taking the article linked above to represent the FAA's "final word" on this very unsettled issue.

A special dispensation for ultralight pilots:
Clip and save:
“My navigation of the national airspace system is informed by the Final Rule published by the FAA in the October 5 1993 edition of the Federal Register (volume 58). The section entitled “Background”, on the third column of page 51966, states that the language of FAR 91.157(a) as it existed immediately prior to that time, which included the phrase “Within the lateral boundaries of the surface areas of … Class E airspace designated for an airport”, would not include any Class E airspace located above the floor of any 700’ or 1200’ AGL E5 transition area that was defined to overly the surface-level Class E airspace. Thus the FAA felt there was a need to modify the phraseology of FAR 91.157(a) to include more airspace. The language was changed to read “within the airspace contained by the upward extension of the lateral boundaries of the controlled airspace designated to the surface for an airport".  I note that FAR 103.17 was never similarly modified and still contains phraseology identical to the earlier phraseology of FAR 91.157(a), and thus according to the logic of the October 5 1993 Final Rule, FAR 103.17 does not require prior authorization for ultralight flight in any surface-level Class E airspace (E2, E3, E4 airspace), so long as said flight is conducted above the base of a 700’ or 1200’ AGL E5 transition area that is defined to also cover the area covered by said surface-level Class E airspace.”
Clip above this line ............................................................................
This is completely absurd, of course, but who are we to argue with the FAA?
Consider for example FAR 91.303(c) prohibiting aerobatic flight "within the lateral boundaries of the surface areas of... Class E airspace designated for an airport"-- does this also not apply above 700' in most surface-level Class E airspace?  And likewise FAR 101.33(a), the prohibition on operating unmanned free balloons below 2000' AGL "within the lateral boundaries of the surface areas of... Class E airspace designated for an airport" -- does this also not apply above 700' in most surface-level Class E airspace?
For a saner view, see this answer.
A piece of trivia-- during the rule-making process leading up to the 1993 "alphabet" airspace re-designation, at one point the plan was that when the airspace formerly known as "control zones" (which in most cases extended up to the base of the Continental Control Area at 14,500' MSL) where converted into surface-level Class E (and higher) controlled airspace, the surface-level Class E airspaces would have specific defined ceilings comparable to what was planned for the Class D airspaces.  But, that was never implemented-- the plans changed before the Final Rule was issued.

This relates to questions 2, 4, and 6.

Link that opens directly to relevant page in Federal Register: https://tile.loc.gov/storage-services/service/ll/fedreg/fr058/fr058191/fr058191.pdf#page=217 .  See text in third column stating "At airports without an operating control tower, the Class E airspace extending upward from the surface would technically terminate at the base of the overlying transition area (700 or 1200 feet AGL).   Effectively, the airspace within which SVFR operations could be authorized would be significantly reduced, resulting in a severe limitation on SVFR arrival and departure operations at those airports."

Taken in context with the language of FAR 91.157(a) as it existed at the time (see body of the present answer), this implies that someone in the FAA thought that the "within the lateral boundaries of" language did not imply a column of air extending upwards indefinitely.

18-1-2. CLASS E SURFACE AREAS
Class E Surface area extend upward from
the surface to a designated altitude; or to the adjacent or
overlaying controlled airspace

103.17 No person may operate an ultralight vehicle within Class A, Class B, Class C, or Class D airspace or within the lateral boundaries
of the surface area of Class E airspace designated for an airport

*** The within the lateral boundaries = this means the area inside the dashed magenta.
The dashed magenta line does not exist past certain altitudes. = to a designated altitude; or to the adjacent or overlaying
Example-
If referring to a non towered airport, the dashed magenta line (lateral boundary) stops at 700' agl. (Its all Class E, it's now Class E 700' boundary. This means there is no longer a surface area boundary past that altitude)
You can fly over the dashed magenta above 700' agl because thats the "designated altitude" and its no longer a "surface area". It is now Class E 700, then that lateral boundary stop at Class E 1200.
Below 700' agl you would be crossing the dashed magenta (lateral boundaries) that exists below 700' and operating within.
Cant remember where I got this from, helps explain rather well.
Class E Airspace:
Controlled airspace is officially defined by exclusion, which often does not tell you much. By that
reasoning, Class E airspace is controlled airspace that is not Class A, B, C or D or G (explained
below) airspace. Not too helpful, but you can be sure that there is a lot of Class E airspace, so
much that one could think of it as "E" for Elemental or Everywhere airspace, the airspace out of
which all other types are carved. It is the filler that fills in under Class A, ie; below 18,000'
msl, and between Classes B,C and D and over the top of Class G (described below). Its volume is
vast. If we ignore the upper cover of Class A airspace, it is safe to say that there is a lot more
E than all the other kinds combined. Ultralights fly freely in most of Class E space with one
exception described below.
Class E almost always has one of four lower limits: those being surface, 700' AGL, 1200' AGL, or
14,500' MSL
(discussed below). Most of the country has a Class E lower limit of 1200' AGL. Where it drops to
700' AGL it is shown by a broad magenta line with a fuzzy side.
The fuzzy side is the side where the floor of Class E is 700' AGL. The floor may also drop to the
surface where it is called "Class E Surface Area". A dashed magenta line indicates the boundary of
the Class E Surface Area. So on the image, you can see the dashed magenta line
around the airport indicating the outer boundary of the Class E Surface

Class E Surface Areas and Floors
Area and outside of that a meandering wide magenta line showing where the floor of Class E goes
from 700' agl to 1200' agl as you go away from the airport.
The major exception to ultralights being allowed in Class E Airspace is: FAR, Part 103.17 -- "No
person may operate an ultralight vehicle within [snip] the lateral boundaries of the surface area
of Class E airspace designated for an airport unless that person has prior authorization from the
ATC facility having jurisdiction over that airspace."
"Class E Surface Area," looks much like Class D but only in dashed magenta, not dashed blue. Notice
there is no upper altitude number within a box like there was in Class D. The question then
becomes... what is the upper limit? How is this airspace boundary defined?
The FAA has a definition for "Class E Surface Area" that includes an upper limit. It is the height
of the surrounding Class E floor just outside the lateral boundaries of the Class E Surface Area.
For example in the image (above), the upper limit of Class E Surface Area is 700' agl meaning
you could fly your ultralight vehicle over this airport but you must remain at least 700' agl as
you  fly over.
fly over.

In all my time flying through Class E airspace that extends to the surface, I have never encountered a dashed magenta line. My understanding is that these exist on the two-dimensional sectional chart only, and do not have any vertical dimension.

@JWalters they are depicted as a two dimensional area on the chart, but they have height, and when associated with an airport, which is the only instance that Class E begins at the surface, they have a volume by virtue of their extension upward to the underlying controlled airspace. Most of the time that airspace is Class E, beginning at 700 feet AGL.

Hi Mike.  We've gone round and round about this on another forum.  This is just not accurate at all.  Specifically the statement "The FAA has a definition for "Class E Surface Area" that includes an upper limit. It is the height of the surrounding Class E floor just outside the lateral boundaries of the Class E Surface Area."  Admittedly the some of the FAA definitions are written in rather obtuse language-- hence my very long answer exploring that language--- but your interpretation is not correct. I recognized your argument long before I noticed the name at the end and it just won't fly.

As someone else said in another comment on another related forum https://aviation.stackexchange.com/questions/35407/may-aerobatics-be-performed-over-a-controlled-airport, "within the lateral boundaries of" has no implied altitude restriction. That term, in English, implies surface to outer space".  Or are we to believe that they were only banning aerobatics below 700' AGL, as far as the Class-E-to-surface is concerned?  Something to think about--

Noting that "within the lateral boundaries of the surface area of Class E arpce dsgntd for an airport" appears in both FAR 103.17 and FAR 107.41 (pertains to Small Unmanned Aircraft), one could use one of the on-line forums for commercial drone operators a sounding board for ideas like "we're fine to overfly Class-E-to-surface surrounding an airport at 701'", or "the dashed magenta lines not in effect if the wx is above standard Class E minimums".
Read more: http://forum.hanggliding.org/viewtopic.php?t=19101&start=180#ixzz5TjJFbVclSmall Unmanned Aircraft

Those forums are well populated and would definitely give you some rapid feedback.  Examples of such forums-- https://commercialdronepilots.com/ , https://inspirepilots.com/forums/certified-uav-pilots.16/ -- there are many more. The resulting feedback would allow this answer to be improved.

Related-- some additional "ammunition" in support of your argument-- (or not) -- https://aviation.stackexchange.com/a/86990/34686

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What is this loose object above the flap fairing?
During my flight, I noticed a loose object above one of the flap fairings. You can see it in this video: https://i.sstatic.net/yxh62.jpg.
Is it normal or should I notify the airline?
People pointed out to a similar question, but A) I'm not sure it's the same rubber; B) it's a different airplane model, C) the behavior of that rubber (if my footage shows a rubber indeed) is not the same, D) the other question asked what that rubber is, whereas my question is specifically about it being loose. I'm no aviation expert nor an engineer, but it doesn't feel right to me that it's loose.

@quietflyer Probably this one: What is this small black rubber strip protruding out of the flap track fairing?

@OferZelig Could you add the tag for the aircraft type you were flying in?

@quietflyer well that doesn't help much...

@Bianfable I've added the tag (it was an Airbus A350-900)

@Bianfable I edited the question to explain why I don't think it's a duplicate

@J.Hougaard I edited the question to explain why I don't think it's a duplicate

It’s a neoprene seal which may be a little loose there at the end like that and made to flutter by the flow of the boundary layer air around it.  That’s a common part used sometimes around the edges of enclosures eg flaps, landing gear fairing doors, etc. 1) to prevent scuffing or abrading parts during operation and 2) to seal out boundary layer flow, reducing drag.  You can notify the airline but the problem is benign and will not affect scheduled operations.

Thanks for your detailed answer, I notified the airline by email since you advised that it's benign.

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How do flight attendants know seat belts are fastened?
I'm a simple curious traveller. I always see flight attendants passing by the seats checking whether belts are fastened. They pass quickly and without looking carefully at each single person, to see if the seat belt is fastened.
I stared at them and found them looking at some specific point on the ground that could possibly tell them with a glance, but I couldn't figure out what was that point.
Can somebody explain how do they check the seat belts status?

Today I learned that the in-flight safety belt check is referred to as "The Crotch Watch" and the "Groin Scan"

You can perceive a lot in your peripheral vision. A quick glance can be enough to see that the buckle is in place.

They look at hundreds of seat belts per day, eventually you get good enough that a "passing glance" is sufficient to decide if it is buckled properly or not.

@ratchetfreak I noticed in many times, the buckle is hidden from the steward vision (hands, book, tablet, etc.) and they still can know if it is fasten or not.

@RonBeyer if it is really with just direct eye vision, then it is really amazing.

This is another question with the answer "MkII eyeballs" (That's the upgrade from MkI eyeballs, with more specific training in what they're looking for)

@NadjibMami they cannot possibly know if it's buckled if the buckle is completely obscured from vision. If they're doing proper checks they will stop and ask you to show them that your buckle is done correctly if they cannot see it.

X-ray vision :D

Funny :) - Having some technical support was my intuition (which is technically feasible). But yea, maybe "they are not properly checking" is the easiest answer.

Found this relevant question: https://www.quora.com/In-aircrafts-why-is-there-no-indicator-to-say-whether-seat-belt-is-fastened-or-not-for-every-seat

They're also checking for tray tables and debris (eg bags) on the ground that would be dangerous in a dark/smokey plane with people panicking trying to get off, hence the looking at the ground and catching your belt status in peripheral vision.

The truth is they can't "know" without checking your seat belt and buckle individually, which they typically don't have the time or interest to do unless they suspect something. It's not difficult to pull the seat belt snugly across your lap and tuck the buckle under your butt cheek and make it look like you're 'securely fastened', at least if you're not in an aisle seat. But there's no excuse for planes not to have had mechanical detection for this (just like cars) for the last 30 years or so; airlines are just cheap.

@TylerH thanks for the addition. Someone suggested that belt-fastening should nowadays become a common sense, so every traveler should automatically think of fastening their seatbelt while seated. They are further reminded during the flight by the attendants and the signal on top of their heads, so a human check is after all not even necessary (anymore). Regardless of how reasonable is this thesis (you mentioned that such a system is already well integrated into cars), I think a small mechanical check would still be a viable addition that could potentially prevent troubles.

Flight attendants check that seatbelts are fastened the old fashioned way - by looking and confirming each passenger has correctly fastened them. There is no indicator anywhere around each seat which indicates as such.
If the attendants is doing this "..quickly and without looking carefully at each single person" then they are [possibly] not doing their job correctly. That they can do it swiftly, is not the same thing; like all things if you practice it every day you can do it "quickly and efficiently" without compromising safety.

More than 2 years later and after travelling several times more, I ensure that some attendants are not really ensuring everyone's belt is fastened. I saw belt-unfastened seats being passed past without notice, even though visibly checked.

In addition to Jamiec's correct answer, I would add that you can tell when a seat belt is not fastened even if the buckle is covered by someone's shirt (or belly!).  You can normally see the segment of belt alongside the person's hip and it's easy to tell if that piece of strap is under tension or not.  Some people actually try to disguise the fact that their seat belt isn't fastened by covering up the buckle, hoping to avoid detection.  By looking for the strap alongside their hip, I can avoid asking to see the buckle itself.
Of course, if I can't really tell by a quick glance, I ask them to show it to me.  It's very rare that anyone actually takes offense to this.

Thanks for the useful addition. Maybe one day my imagined solution for a guaranteed check will be implemented :)

@NadjibMami even if the seat belt check could be automated, there are additional things the flight attendants need to check.  Baggage stowed, seats upright, tray tables, phones stowed, etc.  A walk through the cabin would still be necessary.

Sure, that passage is still important, it makes the flight more convivial to my opinion. Only the careful seat belt check would be saved.

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Are seaplanes with floats more aerodynamically efficient (less drag) than planes with tires?
After getting an unexpected answer from What's the fastest fixed gear airplane? post, I wonder if seaplanes with floats are more aerodynamically efficient (less drag) than fixed tire airplanes?

I have much seaplane flying and can tell you that, in general, floats will take about 15-30% off a landplane's cruise speed. For example, a DeHavilland Beaver will cruise at low altitude on wheels, about 115kt. On floats, it'll do about 95 kt.
A Cessna 180 on wheels cruises around 130kt at low altitude, and on floats about 115ish.
The floats cost speed, some payload, that is, payload is reduced by part of the weight of the floats (some credit is taken for the lift they produce, so if they weigh, say, 500lbs, the deduction from the useful load may only be 200 lbs).
And they move the vertical axis neutral point forward, reducing yaw stability (weathervaning tendency) and usually require added fin area at the tail (later Cessna 180s, and 185s, have an oversize dorsal fin in the basic aircraft, and don't require any additional fin area when on floats).

A decisive answer!

@Fattie Yeah, I was about to say, that's a damn good answer. Can't be more complete. Also, i've always been curious about that. I figured if I flew over water, I'd only use floats!

Fixed landing gear is draggy -- the wheels, tires, and brakes are very draggy -- some of that can be improved with wheel pants and other fairings.
Floats can be shaped somewhat aerodynamically, but they still have large frontal and wetted area.  They usually have a step with no aerodynamic fairing.
The structure to hold the wheels or floats can be thought of similarly -- struts and wires can be streamlined, but they are still draggy.  Having two or three times as much of this stuff (like a floatplane will) will certainly cause more drag.
I think the question you're really after is a comparison of a like-to-like aircraft...
Would a Schneider Trophy aircraft have been faster if it was not required to be a seaplane and instead was fitted with fixed landing gear.
I am confident saying yes, a Schneider Trophy aircraft modified to have reasonably cleaned up fixed gear would have less drag than the floatplane version.
That said, the benefits of retractible landing gear were well understood at the time.  A comparable land plane racer at the time would certainly have had retractible landing gear.
If they had had a technology for retractible floats, the Schneider Trophy aircraft would have used them.
Edit:
Floats and the structure that support them are also heavy.  More weight means more induced drag.
A simple fixed gear would certainly be lighter than floats.  A retractible gear would be heaver than fixed -- but still lighter than floats.
At Schneider Trophy speeds, accepting the weight penalty (compared to fixed gear) of retractable gear would certainly be worth the drag benefit.  But either solution would be lighter and cleaner than floats.

One of the innovations in Schneider Trophy competitors was eliminating stuts and wires.  Even a fairly fat, but well streamlined single float leg was a lot cleaner than a narrow leg with support wires.  Likewise, cantilever wings and tail surfaces came along pretty early; innovations from this race series fed into military pursuit aircraft a decade before the conflicts in Europe brewed up into World War.

Another element of Schneider Trophy aircraft was they were optimised for high cruise speed & also had relatively high stall & V2 speeds. They had quite extreme fixed-pitch propellers meaning they weren't remotely efficient at low speeds leading to very long take-off distances. At the time there were few paved runways & wheeled undercarriage for these aircraft would have had to have been built somewhat more ruggedly than other land aircraft due to time & speed on the grass or gravel.

At least the large float area could be used for cooling, reducing the need for (also very draggy) radiators. Still, floats clearly produce more drag than wheels. +1

Also note that many float planes and flying boats had (semi)retractable outrigger floats for a reason: drag reduction.

One point not addressed is that float planes used the floats as fuel tanks, which enabled the fuselage to be smaller and the wings to be slimmer, and improved stability. Yes, it is totally counter-intuitive to weigh down the floats with fuel, but the floats have to support the same all-up weight when on the water, wherever you put the fuel.

If the majority of the floatation is provided by the hull (ie, a flyingboat rather than a seaplane) then certainly adding wheels would make it worse. But such hulls are a compromise between aerodynamics and hydrodynamics, meaning it's not especially good at either.
On the other hand, if you start with a regular plane and add floats (replacing normal undercarriage), that will clearly make it a lot worse, since they have enormous frontal cross section and they move the aerodynamic drags centre down, requiring other charges to compensate. But at least in this case you can add wheels that barely protrude from the bottom of the floats, with minimal impact.
You might also like to consider skids for landing on packed snow or ice. These are usually fitted in addition to wheels, and can often be retracted along with the wheels, so there is minimal difference to the drag once airborne.

Would it be practical to have a plane whose hull was not designed to float when stationary, but could use retractable waster-ski-like outriggers for takeoff and landing?

Skis aren't buoyant, so they will sink as soon as your speed drops below minimum aquaplaning speed; I'm hoping the hull is buoyant! That would only be useful for landings, and you would probably want to park on land.
If you mean some kind of retractable floats, they would require much larger bays to retract into, impairing the internal capacity of the aircraft. Or you could have some kind of hybrid that partially retracts, with their remaining exposed surfaces shaped to match the aerodynamic profile of the aircraft.

A flyingboat would probably be simpler - no moving parts - and not much worse aerodynamically; modelling would be needed to check this.

My limited understanding of hydrodynamics and aerodynamics would suggest that to minimize drag in the water during takeoff one would want a hull shape that would push fluid to the sides, which would reduce the amount of lift the hull would provide once airborne.  Additionally, a seaplane hull needs to be wide enough at the bottom to minimize draft.  If a seaplane could sit lower in the water when not taking off or landing, it could use a hull shape much closer to that of a normal airliner.

@supercat Prior to transitioning to hydroplaning, you start with a basic boat design ("U" shaped cross-section with a sharp bow), but modified to balance buoyancy and hull drag both as functions of vertical displacement, and lift as a function of speed, so that you can smoothly lift out of the water on the way to hydroplaning. Once you've transitioned to hydroplaning you want a fairly flat bottom that can skip over the surface; simple skis would work for that.

Simple skis would work, but I would think they would have the same drawback in the air as fixed wheels.  I would think that retractable skis could offer the same advantage as retractable wheels.


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How do friendly stealth planes locate each other?
Stealth planes can't be seen (very well) on radar. So one stealth plane should be unable to locate another stealth plane on its radar and vice versa.
So how would one stealth plane locate another, friendly, stealth plane?

If someone knew the answer, they would not post it here.  If they don't know the answer, they will be guessing.  Therefore, voting to close. Sorry.

@Simon I disagree. Surely stealth technology can be spoken about in general terms without affecting state security.

@Ben Yes, but then it would be guesswork and therefore of no use.

@Simon: Stealth can only be maintained when no radiation is emitted. Radar is not an option, and the premise of the question is wrong.

@Simon You seem to be assuming that there is zero public information about stealth aircraft. This is not at all true. There's plenty of scope for non-speculative answers.

@DavidRicherby I'm assuming nothing except that the actual methods used will not be in the public domain.

The question can be reformulated asking for which possible technologies could allow stealth airplanes to communicate/detect each other will not be opinion based and will offer and idea of the possibilities the airplanes will have.

I would have considered closing the question but aviation is different.  There are people and enthusiasts who just like learning and talking about topics like this.  On the other hand chat would be my first choice to post this question.

The accounts that have been published of F-117 missions indicate that they generally flew single-ship, rather than formation missions, and it seems probable that B-2 aircraft do likewise.  So in a sense, the important question for those aircraft isn't "how do I know where (another stealth aircraft operating nearby) IS," but "how do I ensure that we don't collide with each other," and that's a much easier question.
There are several ways to "deconflict" multiple aircraft operating in relatively close proximity that don't involve seeing or painting each other on radar.  One aircraft operates at one altitude, another operates at a different altitude is one method, while timing over a point (you get to waypoint ABC no later than 19:00:00, and I'll be there no sooner than 19:00:30) is another, and physical separation (you stay west of this longitude line, I stay east) is another.  In practice, all of these can be combined to ensure that "your" mission is deconflicted from "my" mission.
Of note, it isn't only stealth aircraft that will use tactics like this.  Helicopters operating blacked out at night don't have air-to-air radar, and even with NVG's, the ability to see each other isn't reliable enough to plan on "see and avoid" and the "big sky theory" to preclude a mid-air, so they'll use some fairly detailed deconfliction plans incorporating separation in space and time.
More recent stealth aircraft such as the F-22 and F-35 are reported to have datalink capabilities so that they can "see" each other without radar, by means of an encrypted, stealthy broadcast of each aircraft's position and track.  This would allow multiple aircraft to fly in formation (not close formation, but perhaps 1000' or more apart), and to work with other aircraft equipped with the same datalink, so even though the bad guys would only be able to see the non-stealth aircraft on radar, the friendlies could all see each other.

From the book "Skunk Works": the technology now exists to preprogram computerized combat missions so that our stealth fighters could fly by computer program precisely to their targets (end quote). It sounds like the routes can be set in advance to prevent airspace conflicts.

I'm guessing stealth missions can follow a program closely.  But in the event of an attack I feel like any "pre programed" route is going to poop pretty fast.

The disadvantage of datalink is that passive radar is a thing. The stealth aircraft also needs to limit it's broadcasting, otherwise the enemy could detect it by simply listening. Even if they are unable to decipher the data, the transmitter can be located with directional antennas and frequency and characteristic patterns can be used to identify the side and guess the type.

So I guess preprogramming is for the "stealth" part, and datalink is for when the cover is already blown?

I know from the methodology of wireless trilateration you need a LOT of signals to determine position with any accuracy, and a very finely-calibrated set of receivers, not to mention knowledge of the broadcast antenna so you know its propagation pattern. The simple "datalink" packets described might show up but be virtually impossible to pin down, especially if they're traveling at high speed when they emit them. At best you could get a very general idea of what region they're in.

@DavidW: That is precisely how the Serbs managed to shoot down one F-117. It flew the same, pre-planned route every day, and they figured out the pattern quite quickly. All they needed to do the next day was to point their guns at the right spot in the sky and fire at just the right time.

@JanHudec - Spread spectrum transmission can help here; if the datalink transmits in parallel on many different frequencies, you can reduce transmission power, increase fault-tolerance of the encoding to compensate for noise, and end up with the same data rate at much lower power especially on any specific frequency that a passive sensor might be tuned to. This can be combined with EW measures such as having a Growler literally run "interference", jamming the listening stations with broad-spectrum radio noise while the stealth craft are operating.

@RalphJ you mention loose formation, but tight formation is an option too, maintaining relative position by visual reference.

To not be detected, it is essential to suppress any radiation coming from the stealth airplane. Therefore, radar is not an option and only passive sensing is possible. When stealth aircraft fly in formation, they keep visual contact and avoid IMC flight.
To enable ship-to-ship communication, the F-117 has an optical datalink which uses red laser light to transmit the pilot's voice to another F-117 flying nearby.

Is it not possible to transmit low power spread-spectrum RF, that looks like background noise unless you have the key to tell you where and when to look? It seems like there should be a compromise to give up a bit of stealth in order to communicate, but still be stealthier than conventional craft (because it's still hard for an enemy to detect that you're an active transmitter and not noise).

@cpast: You are correct, and that is the thinking behind the Multifunctional Advanced Data Link. The laser link was chosen as the most stealthy and least likely to be disrupted solution, but spread spectrum links are stealthy enough.

How does the plane know where to aim the laser for the datalink transmission? Does it acquire the point of aim visually?

@user31389 Honestly, I don't know.

There is no optical datalink over laser for F-117 pilot communications. Comms on that plane was my primary job and I can assure the readers it had no such thing.
The comment about single ships was about right - we had pre-programmed routes and due to autopilot and autothrottle the goal was to keep the airplane on route on time. The comms antenna were extendable, but during combat ops in foreign territory there were retracted and completely shut off all comms.
The idea of skin mounted antennas started to come around but it had not been implemented by the time I left late in 1996. For the OCIP III ring laser INS with GPS sync it was implemented on the backbone of the plane.

You can turn this into a good answer by just addressing the "how" in the question, and making whatever points you feel need to be made.  It is also  correct to qualify an answer (by addressing only one model)  since not all "stealth" aircraft are alike in their stealth.  (F-117 is a different generation of stealth, as you are more keenly aware than most).  Glad to see a Stinkbug guy here, welcome! :-)  No need to address any of the other answers. Each answer is "supposed" to stand on its own, in the SE method.

The simple answer is that the primary communication is visual when communicating plane to plane. That is established and is part of the ops protocol. To be clear however, normally stealth missions are not flown in a conventional formation.
Aircraft such as the B-2 employ from sat transceivers, datalink and even VLF. Transmit capability is in place for most bands, except to my knowledge not for VLF.
It is not well known whether stealth aircraft have been operationally deployed with metastatic radar, but such radar constellations have been developed for battlefield use. They are effective against low observable targets. A passive metastatic receiver could be deployed on an aircraft such as the B-2.
What is known is that a variety of optical com methods have been employed to permit plane to plane direct communications, at distances of less than 20nm. At first these relied on incoherent sources, but several systems have been tested using coherent sources and tested for data and voice. One such system was employed operationally configured to handle voice. There has been no recent open literature that I have seen recently which would indicate the deployment and operational reliance on those systems.
BTW, selection of the laser frequencies can be done to limit the intercept range of optical communication. For example, using a frequency absorbed by methane can create a good signal to noise at altitude, and the attenuation gets greater closer to the ground. The effect is two-fold as the emissions are absorbed at altitude (enroute) but the attenuation is much greater at lower altitudes where methane is more abundant.
There are open briefings which reference plane to plane optical (laser) com being used during the enroute phase of what are normally long aircraft missions.
Mission separation strategy is spatial and temporal, and that doctrine has not changed. Temporal separations of less then two minutes have been openly documented.
To summarize, assuming a given phase of a sortie is flown in a stealth mode, it is not possible to utilize "radar" in the sense that the stealth's aircraft radar transmits and receives it's own transmitted returns. To do so would reveal the aircraft's location. However, metastatic radar techniques could allow passive reception of other radar transmitters, and a situational rendering. In absence of that capability, the likely scenario is that datalink from other aircraft, such as AWACs, would provide battlefield awareness, and additional metadata to identify friendly aircraft. Even with metastatic radar, there is not ability to be completely autonomous.
When sorties are convergent, visual identification and separation techniques are used.
Finally coherent optical com are available on some aircraft for plane to plane communication.


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How are evolution and variants of aircraft considered by certification authorities?
Recently, both Boeing and Airbus launched "new" aircraft as a modernized version of their respective best sellers (B737MAX and A320neo). To narrow the question down, I will consider the A320 and the FAA, but the same goes for the B737 and almost certainly many more aircraft families, and I hope for other certification authorities as well. I'm not really clear on the difference between type, model and series but I think the difference may be relevant for the certification process. Perhaps highlighting this difference is enough to answer the question.
Wikipedia lists multiple variants of the A318. Do the A318-111 and the A318-121 require different certifications?
Moreover, when adding a variant to the family (here adding the A318 to the family composed of the A319, the A320, and the A321), do the certification authorities consider this aircraft as a totally new one or only as a variant of existing one (many shared components)? Can this consideration lighten the certification process?
Is the new version (A320neo) considered as a new aircraft by the certification authority and does it thus require the same set of tests as a new aircraft?
In short, when a new aircraft is an evolution or a variant of a previous one, how is it considered by certification authorities?

The Certifying Authority for Airbus is EASA, for Boeing it is the FAA. So either pick the A320 and EASA or the 737 and the FAA.

My guess is that the certifying authority ignore marketing labels like -MAX or -neo and take each aircraft on a case by case basis. I expect manufacturers are required to notify the certifying authority long in advance if there is any change that might affect certification.

@DeltaLima the FAA rely on DGAC work (not EASA) for the certification process, and add some requirements

@ManuH The French DGAC is no longer responsible for the certification of aircraft types. When the A320 was developed they were but EASA took over.

@DeltaLima Indeed. I doubt the freshness of the reference I linked in my previous comment. It may have been written before EASA was created (2002).

I'm not really clear on the difference between type, model and series

A type is the basic airplane design, like the A320 type, which includes the A318, A319, A320, and A321. A series is the basic version of the type, like the A320-100 series. A model is the specific configuration of the series, like the A320-111 model. The model may be mainly a different engine, or also include other changes specific to a customer.

Do the A318-111 and the A318-121 require different certifications?

No. Airplanes are generally certified at the type level, which also covers all series and models of that type. However, each new model does have to be certified, which means it must be included in the applicable type certificate.

When adding a variant to the family, does the certification authority consider this aricraft as a totally new one or only as a variant of existing one?

As above, a new variant will generally be considered a new series within that family (type), and so will be included in the applicable type certificate.

Is the new versions (A320neo) considered as a new aircraft by the certification authority and thus require the same set of test as a new aircraft?

As with new variants, new versions like the A320neo are usually included in the same type certificate, but this can vary between authorities. For example, it looks like the UK's CAA has separate type certificates for the 747-100/200 (FA5) and the 747-400 series (FA44). The FAA (A20WE) and EASA (EASA.IM.A.196) have one certificate for the whole 747 series.
The level of work required to certify a new model or series depends on the degree of differences from the previously certified configurations. If something was previously certified, then this will probably apply on the new model. Of course there will be more differences for a new type than just a series, especially when changing other aspects of the design, as with the A320neo. The decisions on what needs additional work to certify are specific to each situation.
As others have mentioned, there are multiple certifiying authorities around the world. An aircraft is first certified with the authority in the country of manufacture (EASA in Europe, FAA in the US, CTA in Brazil, etc.). When an aircraft is to be operated in another country, to avoid restrictions on operation, the authority in that country must also certify the aircraft. The authorities tend to have agreements with each other to make this process easier.

Nice clear answer with well suited references. Thanks for the multiple clarifications, including your last paragraph.

Variants are seen as a modification of an existing airplane design, and only the changed parts need to be certified. The real advantage is that the set of rules the certification has to comply with are frozen at the time of the initial application, so any additional rules that had been added later are not relevant. This is called "grandfather rules".
This definitely lightens the certification process and makes it less complex, but makes the safety enhancements, which were added after the type was certified first, optional for newly built airplanes.
The A318, A319 and A321 are all variants of the original A320, and the subtypes you name differ in their engine. Each engine option has to be certified separately, but all certification work not related to the engine can be shared between the variants. Additionally, the purchaser will select his options for instrumentation and furnishing, chosen from a limited set of previously certified options.
When a new version is introduced, the public will get a very different message from the airplane maker than the certification authorities. For the PR, the new variant is all new, but the FAA and the EASA will hear that almost nothing changed. In the end, it is similar to corporate profits: The shareholders get told the company made oodles of money, while the tax authorities will be presented with a bleak picture of mainly losses.

I like the comparison to corporate profits :-)

BTW, rules that are added later are often relevant. Some rules apply to aircraft being built,being registered or operated , not only to certification.

@DeltaLima: These are mostly operational rules, and they must be followed by all. Please correct me if I am wrong, but I learned that when FAR/JAR 25 is concerned, the rules are those at the time of initial application.

That may be true, I am trying to find out how that works with FAR/JAR 25. I was thinking about changed parts like  instruments that need to comply with current standards. How does that work for interior design? I can't imagine you can deliver a 737MAX that only complies to the emergency lighting and smoke/flammability requirements for aircraft cabins of 1967.

@DeltaLima: If you fly a pre-transponder era aircraft, you are still legal in VFR with original equipment, but must not enter controlled airspace unless an adequate transponder is fitted. To calculate landing distances, a B-737 needs only 2 sec between touchdown and application of brakes, but an A-320 must wait 3 sec - the difference in FAR-25 between the sixties and the late eighties. Just one of many examples for different treatment due to grandfathering - even with the newest 737s.

First of all, the Certification Authority for Airbus is EASA, for Boeing it is the FAA.
Let's look at the A320 family. All variants, A318, A319, A320 and A321 of all series are on the same type certificate EASA A.064(data sheet). For the areas where these aircraft are different from the original A320, Airbus has demonstrated that they meet the requirements specified in the regulations. The A320neo will treated the as a new variant of the A320, so only the differences are tested for. Note that this is by no means a small task if you are introducing a number of significant changes which affect the performance of the aircraft.

The Boeing 737 type certificate is FAA.A16WE (datasheet).

nevertheless, airbus aircrafts are certified by both the FAA and the EASA. See for example this page focusing on the A380. I guess the FAA certification is needed to fly over the USA

@ManuH Most of the certification is done by EASA. FAA looks at the differences between their FARs and European JARs and addresses those with Airbus. This happens in parallel with the EASA certification.


text:
Why does overspeed cause structural damage or failure when limit load factors are not exceeded?

The positive and negative limit load factors in this diagram are around 4.5 and -1.8, which means the aircraft experiences structural damage when the aircraft is flown beyond these limit load factors, and beyond the positive and negative ultimate load factors, which are around 6.6 and -2.8 respectively, the aircraft would experience structural failure. These excessive loads typically occur when the aircraft is maneuvered abruptly as in steep dives and steep turns or when the aircraft is flown in turbulent air.
I understand it's excessive loads that cause structural damage or failure when an aircraft is flown beyond the positive or negative limit load factors. But what induces structural damage or failure when the same aircraft is flown within the limit load factors?
My best guess is the parasite drag, but the fact that the normal operating range (green) within the limit load factors jumps to structural failure (red) without going through structural damage (orange) leaves me at a loss, because if the cause is the parasite drag, I don't see the reason structural failure should occur without the structural damage being first experienced.

Edited in response to Simon and kevin's advice

My guess is you're asking about the jump from "Caution range" region to "Structural failure" region, correct?

@kevin Correct! That was the gist of my question.

At its current state, it took a bit of guess work on my part to arrive at that conclusion. You may want to edit the question to make it more clear to other readers.


text:
Is reducing V1 a viable option for pilots when the ASDA is shorter than the ASDR?
Let's say you were planning to ferry some passengers and cargo in a jet but after some calculations found out the ASDR (Accelerate-Stop Distance Required) was greater than ASDA (Accelerate-Stop Distance Available) of the only runway available in the airport. Can you address this situation by lowering V1 speed for the flight and thus shortening the ASDR?
This question has a context. People discuss the effect of a runway length on V1 on this post and seem to agree a runway length has no bearing on V1. In other words, however long runway you may have for your takeoff roll, if an engine fails after V1, you should continue the roll and take off, not the least because you never know you might be able to make a full stop without a runway excursion.
Granted what they say is true, is it as well not permitted to plan a reduced V1 speed for the runway whose ASDA is shorter than the ASDA for a maximum takeoff weight flight of the day?

You can't just arbitrarily lower V1. If an engine fails after V1 you still have to have enough rwy to reach Vr then V2 on one engine.


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Why does ATC ask emergency aircraft about fuel on board?
From what I hear, it looks like it's a standard procedure that ATC asks about fuel on board whenever an aircraft declares an emergency. Do they need the information for their own planning purpose or just to pass it along to the fire crew so that they get prepared for possible emergency situations like a fire upon landing?
I thought the second was the correct reason, but the fact that some pilots report fuel in remaining time, not in pounds, gets me confused.

Some pilots may report fuel remaining in time because they do not have the remaining fuel volume to hand.  A PA-28 fueled to the tabs will have 128 litres and that will give a flight time of 3.5 hours.  If the emergency occurs 90 minutes in, the pilot will know that there are 2hrs of fuel left, but will have to calculate the remaining volume - not a distraction you want when you have an emergency

@DaveGremlin At least on all of the PA-28s I've flown, the fuel gauges are marked in volume (which makes sense, considering fuel burn per time varies by throttle setting.) Granted, whether the fuel gauges on the PA-28 should be believed or not is another question...

In most light GA aircraft, the most accurate fuel gauge is your clock.

It depends on the type of situation. If the aircraft is unable to land for some reason, we'll want to know the remaining fuel to get an idea of how long the aircraft can realistically stay airborne. For an aircraft coming in for an emergency landing - as you have already guessed - the reason we ask for remaining fuel is because the fire and rescue personel needs this information. We will also ask such an aircraft if they have any dangerous goods onboard, since we need to pass this info on to the fire crew as well.
During an emergency, we have a checklist to follow that reminds us which information we need to obtain, and who needs to know what is going on. For a tower position, it may look something like this:

As you can see, fuel and dangerous goods are grouped together, indicating that they are essentially the same thing - something that may cause fire, explode or otherwise cause danger during evacuation of the aircraft.

Why is there something blacked out on that sheet?

@Polygnome I'd imagine it's "Inform " and redacted as personal information.

@polygnome Because it is not relevant in this context

Great job on including the check-list, immediately turns this into a definitive answer.

@J.Hougaard is it a name? Just curious - why would that be a name, where the others are a position eg "Watch Supervisor" or "Rescue work leader"?

@BruceWayne It's an intelligence agency. They have to make sure to clean up any evidence of the chemicals used for the chemtrails in case of a crash. :)

ATCs have checklists too? aviation loves its checklists ;)

@cat Well, aviation put checklists into perfection! Even the order of items on most checklists there is well thought out. I wonder how many checklists of checklists are there.

Come to think of it, why have I never heard the tower relaying fuel information to the fire crew when they coordinate on the radio for arriving emergency aircraft? I have listened to many video clips on the emergencies on Youtube but never heard tower saying of the information. Why is that? Is it because the information relayed to the headquarters of the fire crew, not the fire engines?

@lemonincider Could be any number of reasons. Maybe they communicate on a different frequency or via telephone, or maybe the information was already relayed before the start of the recordings you have seen

Could the blacked out part be "Clergy"?

@MikeBrass It should be easy enough to tell. The redaction doesn't look like it was particularly well done.


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Why is mixture lean recommended on the ground?
Mixture lean is typically recommended during the ground operations at sea-level pressure in the POHs of most small aircraft equipped with a carburetor system. Why is it?
I heard some people saying it's because the engine requires less power for the ground operations, but that doesn't persuade me. If it's the amount of power output at stake, we just need to pull back the throttle and reduce power, which we exactly do for taxiing and holding short of the runway. Mixture is about controlling the ratio of fuel and air entering the cylinders, and we change it according to the variations in air density, not the power output required. If that's the case, then why do the POHs recommend a mixture lean for the ground operations at sea level?
Edit) I guess my question is put too poorly for some people to understand easily. Let me try again. Plug foulings happen when mixture is set too rich for the given density altitude. For this reason, many of the GA aircraft's POHs recommend pilots to start leaning mixture from 3000 MSL up. That means as long as the aircraft stay below 3000 MSL, the pilots have no reason to lean mixture, at least for the purpose of avoiding plug foulings. Yet, the same POHs recommend pilots to lean mixture for ground operations even when the elevation where ground operations are performed is below 3000 MSL. So my question is: "Why do the POHs recommend pilots to lean mixture for ground operations even when the density altitude of the ground does not require it?".

Are you considering the large number of airfields (in the western US, at least) where ground operations are conducted well above 3000' MSL?

The question assumes the answer that it is recommended.

The new insurer for the Cessna 210 I fly is only charging \$300 to add me to the policy—instead of $1,300 the old one wanted—so I’ve been reading the 210 manual for the first time in several years. I noticed that the Owner’s Manual doesn’t mention anything about leaning on the ground and gives advice about leaning in the air that has been superseded by newer best practices. I looked in the manuals for the Cherokee and Cessna 182 and they don’t mention anything about leaning on the ground either.
One of the first things I learned when flying the Cessna 182 is to lean aggressively on the ground. There were several flights when I first started flying it that were not able to be started because, on runup, we couldn’t get the mags to clear. After a couple of expensive trips to the A&P, I started leaning aggressively on the ground. At the time, I was too busy with the firehose of information to even think about why I needed to lean, just that it saved a lot of money if I did. This post is an explanation of why you need to lean on the ground—especially older small-bore engines.
Fuel is different from the 1960s and 1970s
The POH for the Cherokee and 182 specify 80/87 fuel. The 210 specifies 100/130 and that 100/130 low lead is also approved. So what do these numbers mean?
Shell explains it really well, (bolding is mine)…

Avgas grades are defined primarily by their octane rating. Two ratings are applied to aviation gasolines (the lean mixture rating and the rich mixture rating) which results in a multiple numbering system e.g. Avgas 100/130 (in this case the lean mixture performance rating is 100 and the rich mixture rating is 130).
In the past, there were many different grades of aviation gasoline in general use e.g. 80/87, 91/96, 100/130,108/135 and 115/145. However, with decreasing demand these have been rationalised down to one principle grade, Avgas 100/130. (To avoid confusion and to minimise errors in handling aviation gasoline, it is common practice to designate the grade by just the lean mixture performance, i.e. Avgas 100/130 becomes Avgas 100).
Some years ago, an additional grade was introduced to allow one fuel to be used in engines originally designed for grades with lower lead contents: this grade is called Avgas 100LL, the LL standing for ‘low lead’.
All equipment and facilities handling avgas are colour coded and display prominently the API markings denoting the actual grade carried. Currently the two major grades in use internationally are Avgas 100LL and Avgas 100. To ease identification the fuels are dyed i.e. Avgas 100LL is coloured blue, while Avgas 100 is coloured green.

EAA explains the change as well…

Avgas 80/87 is used in low compression ratio aircraft engines, and contains little or no lead (up to .5 grams of lead per gallon is allowed, but none is required). It is red in color, and should not be used in any automotive engine due to a low motor octane number of about 80.
Avgas 100/130 can be used in some automotive engines. It has both research and motor octane numbers slightly over 100. Avgas 100/130 is green in color, contains four grams of lead per gallon, and is becoming hard to find.
Avgas 100 LL (the LL stands for “low-lead”) contains two grams per gallon, half the lead contained in the avgas 100/130 it was designed to replace. It has research and motor octane numbers very similar to 100/130 avgas. The color is blue. This product sometimes has a high level of aromatics.
Avgas 115/145 was developed for high performance piston aircraft engines used in World War II and the Korean conflict. It is very hard to find today due to lack of demand, and is usually only produced on special order. The color is purple.

Purvis Brothers has similar information in metric units:

Tetraethyl lead content:

80/87 Max of .14g/l
100/130 has 1.12 g/l
100LL has .56 g/l

So why do plugs foul?
The engines on the Cherokee and the 182 were designed for fuel that has little or no lead. 100LL has at least four times as much lead as 80/87. At annual there is a substantial amount of lead that needs to be removed from them. On the other hand, the 210 was designed for fuel that had twice as much lead. I don’t recall ever fouling the plugs and they don’t usually have much in the way of lead deposits at annual. I spoke to two other 210 drivers and they don’t recall fouling the plugs either—though they do lean aggressively on the ground.
As Steve Ells explains “The additive in Avgas that helps scavenge the lead in the fuel doesn’t do much until the combustion temperatures get up to around 1,400 degrees F.”
At idle, your engine isn’t generating much heat. If you remember from studying for your PP test, fuel is a coolant. So the more fuel you give an idling engine the cooler it runs. Leaning the engine makes it run hotter—which is what you want on the ground.
Leaning
None of the owner’s manuals I’ve looked at mention leaning on the ground. Probably because, when the planes were manufactured, fouling plugs wasn’t an issue. Fuel prices were low, so saving a half-gallon or so wasn’t that big of a deal. The truth is that a full-rich mixture combined with the low power setting is the perfect scenario for lead fouling your spark plugs and valve stems.
So why don’t instructors teach new students to lean aggressively? Maybe so low time pilots won’t forget to go full-rich before take-off. But if you lean appropriately, this will never be a problem. Lean the plane at taxi RPM (usually 1000 RPM) so that it starts to stumble. Then enrich just a bit. You’ll be able to taxi and do your runup at this mixture setting without worrying about taking off with less than full mixture. As soon as you advance the throttle a couple hundred RPM the engine will stumble. If you tried to advance to takeoff power, the engine would die from lack of fuel, before you even started your take-off run. To avoid the embarrassment of stalling when I leave the runup area, I put my hand on the mixture knob when I make my call to the tower or CTAF. And I don’t take it off until I’ve pushed the mixture to full rich for takeoff. (Or less that that for high-density altitude.)
John D. Collins explains his procedure for high density altitude:

At sea level up to a density altitude of approximately 3000 feet, I would push the mixture full rich for takeoff. At higher density altitudes, I would do a full power run up and set the mixture to achieve the highest RPM as this will give you the most power available. As you climb above 3000 MSL, I would lean the engine for the highest RPM.

How octane numbers are calculated.
From Shell:

Engine knock, which describes explosive detonation of the fuel/air mixture or preignition, can cause severe engine damage and subsequent failure in a short period of time. Anti-knock ratings are expressed as Octane Numbers for those of 100 or less and as Performance Numbers for those ratings above 100. These numbers relate the fuels performance compared to a reference fuel of pure isooctane. Because the anti-knock characteristics are influenced by the air/fuel mixture ratio, ratings are developed for both rich mixture performance and lean mixture performance. Rich mixture settings yield higher octane or performance numbers since the added fuel acts as an internal coolant and suppresses knock. Prior to 1975, both numbers were reported as the grade designation but current specification utilize only the lean mixture rating. Currently, ASTM (American Society for Testing and Materials) specifies five grades – 80, 82UL, 91, 100, and 100LL (low lead). In practice only 100LL is widely available. Production of Grade 80 has generally ceased due to small demand.

I don’t know when 80/87 started to become less available, but Service Letter L185B from Lycoming dated January 19, 1988 states that “Whenever 80/87 is not available you should use the lowest lead 100 grade fuel available.” It goes on to say that, “higher lead fuels can result in increased engine deposits both in the combustion chamber and in the engine oil. It may require increased spark pull maintenance and more frequent oil changes.…Operation at full rich mixture requires more frequent maintenance periods; therefore it is important to use proper approved mixture leaning procedures.”
The Service Letter doesn’t mention leaning on the ground, but it does mention that:

At engine speeds of 1000-1200 RPM on the ground, the spark plug temperatures are hot enough to activate the lead scavenging agents in the fuel which retards the formation of salt deposits on the spark plugs and exhaust valve stems.

Since the leaner the mixture, the hotter the cylinder temperatures, leaning aggressively would help with reducing lead deposits.
The letter provides a method for leaning in the air to reduce deposits.

So to sum up, 1) Plug foulings result when the temperature inside the cylinders is not high enough, 2) The combination of low engine power and rich mixture keeps the cylinder temperature from rising enough for complete combustion of the fuel entering the cylinders, 3) Mixture lean prevents this by keeping the cylinder temperatures high. Thank you.

I have a question for you. I now understand mixture should be leaned on the ground to keep cylinder temperatures high. But doesn't this cause detonations in the cylinders, since the density altitude on the ground still doesn't require the mixture lean? So are we basically causing detonations in the cylinders on purpose to keep cylinder temperatures high?

@lemonincider You don’t get detonation because the engine is running at a very low percentage of power. That’s why you can do your runup leaned out. You are only at maybe 60% of rated power.

This is done to prevent extended running with an overly rich mixture, which can  result in fouled spark plugs, lead deposits, and rough engine running.  It also consumes less fuel.  Before take-off the mixture is moved to full rich to ensure no fuel starvation, and avoid power loss, but on the ground where even if it is accidentally leaned too far, the engine stopping would not be a major safety concern.

Then, my question should be why rich mixture, which requires a mixture lean, happens on the ground, when you don't have that rich mixture when flying at sea level (between 0 to 3000 MSL)?

I'm not sure if I understand your question.  An aircraft on the ground at 5000' ASL could be lean even with the mixture full rich.  Alternatively you could be on the ground at sea level, but the density altitude is -1000', where you would be too rich.  There is an optimal mixture, but we don't normally adjust for that on the ground, and instead just go 1" lean for example, or whatever the POH recommends.  In the air, the pilot would adjust the mixture for best economy or best power, etc. as desired at the cruising level.

Thank you for your detailed explanation but I think you're still missing my point. My question is why we're recommended to lean on the ground whose elevation does not requires mixture Let's say you're taxiing on the field whose elevation is 1000 MSL. Are we recommended to lean mixture by the POH? Yes. Let's say you are flying at the same altitude, which is 1000 MSL. Are we recommended to lean by the POH? No. Why this difference? This is my question.

We want to lean the mixture on the ground to use less fuel and avoid fouling plugs.  Are you asking why does the mixture control go so rich that you would want to lean it when at sea level?  The mixture control gives extra travel beyond optimal mixture for sea level, to be sure you can get it rich enough in all situations.  E.g. the -1000 density altitude I mentioned, or badly adjusted mixture linkage, etc.

I edited my question to address some confusion about my question. Thanks.

Your question appears to be Why should I lean my engine's mixture on the ground (when idling/taxiing)?
To get to the answer let's start with a few assumptions:
Assumption 1: The engine will be operating at 50% power or less.
Assumption 2: We're at or near sea level (Density altitude of 3000 feet or less).
Assumption 3: Full rich is VERY rich - Lots of extra fuel to cool the cylinders during takeoff.
All of these assumptions hold true for your typical GA piston aircraft during typical ground ops (idling/warm-up & taxiing).

Why should you lean an engine for ground operations?
Running an engine at low power with a very rich mixture creates three problems:

Because you are dumping more fuel into the cylinders than can be burned you're getting incomplete combustion: This produces lots of carbon soot that will work its way past the piston rings and make your oil dirty.
Because you are dumping more fuel into the cylinders than can be burned you're blowing some quantity of unburned fuel out the exhaust. That's avgas you paid for that's effectively going to waste.
Because the engine is operating at a low power setting the internal combustion chamber temperatures are relatively low (and reduced further by the excess fuel), which prevents the lead-scavenging agents in your avgas from doing their job. That means lead ash will deposit on your spark plugs, and it will also work its way into your oil just like the carbon soot does.

None of these are particularly good things for your engine, and the solution to all 3 problems is the same: Lean the mixture so you're putting less fuel through the cylinders:

The fuel you're putting in the cylinders will burn more completely, leaving less soot.
You're burning all (or at least most) of the fuel, so less is being wasted out the exhaust.
There is less excess fuel to cool the combustion chamber, so it will reach a temperature where the lead-scavenging agents in the avgas will bond with the lead and carry more of it away in the exhaust rather than depositing it in your engine.

How should you lean an engine for ground operations?
Now that we covered the why it behooves us to think about how to properly lean on the ground.
The general rule is that if you are going to lean on the ground you want to lean as aggressively as possible: Remember the first assumption we made above is that the engine will be operating at 50% power or less, and at those power settings you can pretty much put the mixture knob wherever you want it as long as the engine is running smoothly: You won't hurt your engine.
Lycoming has some recommendations in SI 1497A which is specifically targeted at the IO-360 engines used in some Cessna aircraft (and more specifically toward flight training operations where you may spend a lot of time on the ground at low power), but the recommendations are generally applicable to carbureted engines as well.
In absence of more specific instructions from your engine or airframe manufacturer a simple way to handle leaning on the ground is to lean as aggressively as possible, to the point where the engine will stumble if you try to advance the throttle to takeoff or runup power. This prevents accidentally taking off with the mixture at an inappropriately lean setting, and because it's putting the minimum amount of fuel possible through the engine it minimizes the three problems we're concerned about.

What about the "Never Lean On The Ground!" myth?
A lot of folks are taught in training that you should "never lean the engine on the ground", or that you should "never lean the engine below 5000 feet" - These are myths based on a common bit of advice you'll find in most POH or engine manuals, which usually reads something like this:

For 5000 ft. density altitude and above or high ambient temperatures, roughness or reduction of power may occur at full rich mixture. The mixture may be adjusted to obtain smooth engine operation.

This is not a prohibition on leaning below a given altitude, though it is often simplified to that level for the sake of students: It's easier for most of us operating at low-elevation fields to be taught "Run the engine full rich on the ground" than explain the inner workings of combustion and fuel mixture to a student who is already overwhelmed just trying to start the engine.
The fact that contradicts the "never lean below some altitude" myth is that both Lycoming and Continental advise leaning the engine during cruise flight (i.e. "extended periods of operation") when the engine is producing 75% power or less, and this recommendation is made regardless of altitude (cruising at 1,500 feet versus 15,000 feet makes no difference - what matters is the amount of power you're asking the engine to produce).
Operating on the ground is, as far as the engine is concerned, "cruise flight" (an extended period of operation with little power change) at a low altitude (sea level) and a low power setting (well below 75% power, typically well below than 50% power), so you can safely lean the engine for ground operations provided you remember to return the mixture to an appropriately rich setting before attempting a high-power operation (runup, takeoff, or getting "unstuck" on a soft surface).
In addition by leaning aggressively (to the point where the engine stumbles when you advance the throttle and cannot produce high power) you ensure that you cannot forget to set the mixture appropriately rich for those high-power operations: as the engine will not allow you to reach those power settings it can't damage itself and the worst it will do is stumble, or stop (starved from an overly lean mixture) if you don't retard the throttle.

At high power (takeoff) a richer mixture is needed to provide additional fuel for cooling, and to avoid excessively high internal cylinder pressures.  The high power also leads to sufficiently high exhaust gas temperatures that carbon and lead deposits won't form, despite the excess fuel and incomplete combustion.
At low power (idle/taxi) a rich mixture will result in lower exhaust gas temperatures and produce carbon/lead deposits that foul plugs. Leaning the mixture encourages more complete combustion, this less unoxidised carbon to deposit, and higher exhaust gas temperatures, preventing the deposits from sticking and burning away deposits that are already there.  It also saves fuel.
So the difference is not so much "on the ground" vs "in flight", but rather "low power" vs "high power".

The question has been answered, yet it is incorrect (and slightly silly if you think about it) to say that rich mixtures cool the cylinder because ‘fuel is a coolant’.
Rich mixtures burn slower, so the peak pressure inside the cylinder as a result of the expansion of the gas due to combustion  occurs later when the piston is further from TDC. Therefore, there is a greater clearance volume above the piston at the point of peak pressure so the peak pressure value is lower, meaning the temperature is lower.
This is why the CHT steadily increases as the mixture is leaned from full rich - because the rate of combustion is increasing meaning peak pressure is occurring closer to TDC. At around ‘best power’ mixture, the combustion rate is fastest, meaning the CHT will be approximately maximum. If the mixture is further leaned, combustion slows again including at ‘best economy’, producing steadily decreasing CHTs.
Additionally, if peak pressure is delayed to around 16 degrees beyond TDC, the angle between the con rod and the crank web is closer to ideal giving greater mechanical advantage and less strain on the bearings.
Detonation results from mixtures not rich (nor lean) enough at high power because the peak pressure occurs too close to TDC when clearance volume is small, meaning pressure and temperature become too high. At high power, a very lean mixture would avoid detonation like full rich does because they both slow combustion so that the clearance volume is greater at peak pressure; however, a mixture lean enough to do so would not be possible to achieve with smooth running because of generally uneven fuel distribution systems, nor would the lean mixture contain enough fuel to actually produce full power. For these reasons full rich is used at high power.